Light emitting element and display device including the same

ABSTRACT

Provided is a light emitting element including a first electrode, a hole transport region disposed over the first electrode, an emission layer disposed over the hole transport region, an electron transport region disposed over the emission layer, and a second electrode disposed over the electron transport region, wherein the hole transport region includes a first hole transport layer disposed adjacent to the first electrode and having a first refractive index, a second hole transport layer disposed adjacent to the emission layer and having a second refractive index, and a third hole transport layer disposed between the first hole transport layer and the second hole transport layer and having a third refractive index greater than each of the first refractive index and the second refractive index, thereby achieving high light extraction efficiency and high luminous efficiency.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and benefits of Korean Patent Application No. 10-2020-0007949 under 35 U.S.C. § 119, filed on Jan. 21, 2020 in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference.

BACKGROUND 1. Technical Field

The disclosure herein relates to a light emitting element and a display device including the same, and more particularly, to a light emitting element including hole transport layers having different refractive indices and a display device including the same.

2. Description of the Related Art

Various types of display devices used in multimedia devices such as televisions, cellular phones, tablet computers, navigations and game consoles are being developed. In such display devices, a so-called self-luminescent display element accomplishing display by causing a light emitting material including organic compounds or quantum dots in an emission layer disposed between electrodes facing each other to emit light is used.

In the application of a light emitting element to a display apparatus, there is a demand for a light emitting element having high luminous efficiency and a long life, and development on materials and structures, for a light emitting element, capable of stably attaining such characteristics is being continuously required.

SUMMARY

The disclosure provides a light emitting element having excellent light extraction efficiency.

The disclosure also provides a display device including a light emitting element having high luminous efficiency.

An embodiment of the inventive concept provides a light emitting element including a first electrode, a hole transport region disposed over the first electrode, an emission layer disposed over the hole transport region, an electron transport region disposed over the emission layer, and a second electrode disposed over the electron transport region, wherein the hole transport region includes a first hole transport layer disposed adjacent to the first electrode and having a first refractive index, a second hole transport layer disposed adjacent to the emission layer and having a second refractive index, and a third hole transport layer disposed between the first hole transport layer and the second hole transport layer and having a third refractive index greater than each of the first refractive index and the second refractive index.

A difference between the third refractive index and the first refractive index may be greater than about 0.1, and a difference between the third refractive index and the second refractive index may be greater than about 0.1.

The first refractive index may be in a range of about 1.2 to about 1.7, the second refractive index may be in a range of about 1.2 to about 1.7, and the third refractive index may be in a range of about 1.7 to about 2.2.

The first refractive index and the second refractive index may be equal.

The second hole transport layer may be directly disposed under the emission layer.

The refractive index of the emission layer may be greater than the first refractive index of the first hole transport layer, and a difference between the refractive index of the emission layer and the first refractive index may be greater than about 0.1.

The refractive index of the emission layer may be in a range of about 1.7 to about 2.2.

The first hole transport layer may be directly disposed over the first electrode.

The refractive index of the first electrode may be greater than the first refractive index of the first hole transport layer, and a difference between the refractive index of the first electrode and the first refractive index may be greater than about 0.1.

The refractive index of the first electrode may be in a range of about 1.7 to about 2.2.

The ratio of a thickness of the first hole transport layer to a thickness of the third hole transport layer to a thickness of the second hole transport layer may be in a range of about 0.1:0.8:0.1 to about 0.45:0.1:0.45.

The first electrode may be a reflective electrode, and the second electrode may be a transmissive electrode or a transflective electrode.

The emission layer may emit light having a central wavelength in a range of about 430 nm to about 470 nm.

The ratio of a thickness of the first hole transport layer to a thickness of the third hole transport layer to a thickness of the second hole transport layer may be in a range of about 1:1:1.

The first hole transport layer and the second hole transport layer may each independently include at least one of the compounds represented by Formulae 1-1 to 1-4.

In Formulae 1-1 to 1-4, A₁ to A₅ may each independently be a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, a substituted or unsubstituted silyl group, a substitution or unsubstituted oxy group, a substituted or unsubstituted alkyl group having 1 to 60 carbon atoms, a substituted or unsubstituted aryl group having 6 to 60 ring-forming carbon atoms, or a substituted or unsubstituted heteroaryl group having 2 to 60 ring-forming carbon atoms. In Formulae 1-1 to 1-4, a is an integer of 0 to 5, b is an integer of 0 to 4, and c is an integer of 0 to 6.

The third hole transport layer may include a compound represented by Formula 2.

In Formula 2, Ar₁ and Ar₂ may each independently be a hydrogen atom, a deuterium atom, a halogen atom, a substituted or unsubstituted alkyl group having 1 to 30 carbon atoms, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, a substituted or unsubstituted heteroaryl group having 2 to 30 ring-forming carbon atoms, or bonded to an adjacent group to form a ring, and Ara may be a substituted or unsubstituted aryl group having 6 to 30 ring-forming carbon atoms or a substituted or unsubstituted heteroaryl group having 2 to 30 ring-forming carbon atoms. In Formula 2, a and b are each independently 0 or 1, and L₁ and L₂ may each independently be a substituted or unsubstituted cycloalkylene group having 3 to 10 ring-forming carbon atoms, a substituted or unsubstituted heterocycloalkylene group having 2 to 10 ring-forming carbon atoms, a substituted or unsubstituted cycloalkenylene group having 3 to 10 ring-forming carbon atoms, a substituted or unsubstituted arylene group having 6 to 30 ring-forming carbon atoms, or a substituted or unsubstituted heteroarylene group having 2 to 60 ring-forming carbon atoms. In Formula 2, p and s are each independently an integer of 0 to 4, q and r are each independently an integer of 0 to 3, and R₁ to R₅ may each independently be a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, a substituted or unsubstituted silyl group, a substituted or unsubstituted oxy group, a substituted or unsubstituted alkyl group having 1 to 60 carbon atoms, a substituted or unsubstituted heterocycloalkyl group having 3 to 60 ring-forming carbon atoms, a substituted or unsubstituted aryl group having 6 to 60 ring-forming carbon atoms, or a substituted or unsubstituted heteroaryl group having 2 to 60 ring-forming carbon atoms.

In an embodiment of the inventive concept, a display device includes light emitting elements, each of the light emitting elements including a first electrode, a hole transport region disposed over the first electrode, an emission layer disposed over the hole transport region, an electron transport region disposed over the emission layer, and a second electrode disposed over the electron transport region, and wherein the hole transport region of at least one of the light emitting elements includes a first hole transport layer disposed adjacent to the first electrode and having a first refractive index, a second hole transport layer disposed adjacent to the emission layer and having a second refractive index, and a third hole transport layer disposed between the first hole transport layer and the second hole transport layer and having a third refractive index greater than each of the first refractive index and the second refractive index.

A difference between the third refractive index and the first refractive index may be greater than about 0.1, and a difference between the third refractive index and the second refractive index may be greater than about 0.1.

The first electrode may be a reflective electrode, and the second electrode may be a transmissive electrode or a transflective electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the inventive concept, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the inventive concept and, together with the description, serve to explain principles of the inventive concept. In the drawings:

FIG. 1 is a perspective view of an electronic device according to an embodiment;

FIG. 2 is a plan view of a display device according to an embodiment;

FIG. 3 is a schematic cross-sectional view of a display device of an embodiment corresponding to line I-I′ of FIG. 2;

FIG. 4 is a schematic cross-sectional view of a light emitting element according to an embodiment;

FIG. 5 is a schematic cross-sectional view illustrating a part of a light emitting element according to an embodiment; and

FIG. 6 is a graph showing the comparison of efficiency characteristics in a light emitting element of Example and Comparative Example.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The inventive concept may have various modifications and may be embodied in different forms, and example embodiments will be explained in detail with reference to the accompany drawings. The inventive concept may, however, be embodied in different forms and should not be construed as being limited to the embodiments set forth herein. Rather, all modifications, equivalents, and substituents which are included in the spirit and technical scope of the inventive concept should be included in the inventive concept.

It will be understood that when an element or layer is referred to as being “on”, “disposed on”, “connected to”, or “coupled to” another element or layer, it can be directly on, disposed, connected, or coupled to the other element or layer or intervening elements or layers may be present.

Like numbers refer to like elements throughout. Also, in the drawings, the thickness, the ratio, and the dimensions of elements are exaggerated for an effective description of technical contents.

The term “and/or,” includes all combinations of one or more of which associated configurations may define. For example, “A and/or B” may be understood to mean “A, B, or A and B.” The terms “and” and “or” may be used in the conjunctive or disjunctive sense and may be understood to be equivalent to “and/or”. Throughout the disclosure, the expression “at least one of A, B, or C” may indicate only A, only B, only C, both A and B, both A and C, both B and C, all of A, B, and C, or variations thereof.

The term “at least one of” is intended to include the meaning of “at least one selected from the group consisting of” for the purpose of its meaning and interpretation. For example, “at least one of A and B” may be understood to mean “A, B, or A and B.” When preceding a list of elements, the term, “at least one of,” modifies the entire list of elements and does not modify the individual elements of the list.

It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the inventive concept. The terms of a singular form may include plural forms unless the context clearly indicates otherwise.

Terms such as “below,” “lower,” “under,” “above,” “upper,” and the like are used to describe the relationship of the configurations shown in the drawings. The terms are used as a relative concept and are described with reference to the direction indicated in the drawings. The invention may also be oriented in other directions and thus the spatially relative terms may be interpreted differently depending on the orientations.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the inventive concept pertains. It is also to be understood that terms defined in commonly used dictionaries should be interpreted as having meanings consistent with the meanings in the context of the related art, and are expressly defined herein unless they are interpreted in an ideal or overly formal sense.

It should be understood that the terms “comprise”, “include”, “contain”, or “have” are intended to specify the presence of stated features, integers, steps, operations, elements, components, or combinations thereof in the disclosure, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

Hereinafter, a light emitting element according to an embodiment of the inventive concept and a display device including the same will be described with reference to the accompanying drawings.

FIG. 1 is a perspective view of an electronic device according to an embodiment. FIG. 2 is a plan view of a display device according to an embodiment. FIG. 3 is a schematic cross-sectional view illustrating a part of a display device corresponding to line I-I′ of FIG. 2.

In an embodiment, an electronic device ED may be a small- and medium-sized electronic device such as a smart phone, a tablet, a personal computer, a notebook computer, a personal digital terminal, a car navigation unit, a game console, and a camera. The electronic device ED may be a large-sized electronic device such as a television set, a monitor, or an outdoor billboard. These are merely presented as an example, and thus it may be adopted for other electronic devices without departing from the inventive concept.

The electronic device ED may include a display device DD and a housing HAU. The display device DD may display an image IM through a display surface IS. FIG. 1 illustrates that the display surface IS is parallel to a plane defined by a first direction axis DR1 and a second direction axis DR2 crossing the first direction axis DR1. However, this is an example, and in another embodiment, the display surface IS of the display device DD may have a curved shape.

Among the normal directions of the display surface IS, for example, the thickness directions of the display device DD, a direction in which the image IM is displayed is indicated by a third direction axis DR3. A front surface (or an upper surface) and a rear surface (or a lower surface) of each member may be defined by the third direction axis DR3. The directions indicated by the first to third direction axes DR1, DR2, and DR3 are relative concepts, and may thus be changed to other directions.

The housing HAU may hold the display device DD. The housing HAU may be disposed to cover the display device DD so that an upper surface which is the display surface IS of the display device DD is exposed. The housing HAU may cover a side surface and a bottom surface of the display device DD and may expose the entire upper surface. However, the embodiment of the inventive concept is not limited thereto, and the housing HAU may cover a portion of the upper surface as well as the side surface and the bottom surface of the display device DD.

The display device DD may include a base substrate BS, a circuit layer DP-CL disposed on the base substrate BS, and a display element layer DP-OEL. The display element layer DP-OEL may include a pixel defining layer PDL, light emitting elements OEL-1, OEL-2 and OEL-3 disposed between the pixel defining layer PDL, and an encapsulation layer TFE disposed on the light emitting elements OEL-1, OEL-2 and OEL-3.

The base substrate BS may be a member that provides a base surface where the display element layer DP-OEL is disposed. The base substrate BS may be a glass substrate, a metal substrate, a plastic substrate, etc. However, the embodiment of the inventive concept is not limited thereto, and the base substrate BS may be an inorganic layer, an organic layer, or a composite material layer.

In an embodiment, the circuit layer DP-CL may be disposed on the base substrate BS, and the circuit layer DP-CL may include transistors (not shown). The transistors (not shown) each may include a control electrode, an input electrode, and an output electrode. For example, the circuit layer DP-CL may include a switching transistor and a driving transistor to drive the light emitting elements OEL-1, OEL-2, and OEL-3 of the display element layer DP-OEL.

Each of the light emitting elements OEL-1, OEL-2 and OEL-3 may include a first electrode EL1, a hole transport region HTR, emission layers EML-R, EML-G and EML-B, an electron transport region ETR, and a second electrode EL2.

Each of the light emitting elements OEL-1, OEL-2, and OEL-3 included in the display device DD of an embodiment may have a structure of a light emitting element (OEL, FIG. 4) of an embodiment, which will be described later. The hole transport region HTR included in each of the light emitting elements OEL-1, OEL-2, and OEL-3 included in the display device DD of an embodiment may include hole transport layers having different refractive indices.

FIG. 3 illustrates an embodiment that light emitting layers EML-R, EML-G, and EML-B of light emitting elements OEL-1, OEL-2, and OEL-3 are disposed in an opening OH defined in a pixel defining layer PDL, and a hole transport region HTR, an electron transport region ETR, and a second electrode EL2 are provided as a common layer in all the light emitting elements OEL-1, OEL-2, and OEL-3. However, the embodiment of the inventive concept is not limited thereto, and unlike the one shown in FIG. 3, in an embodiment, the hole transport region HTR or the electron transport region ETR is separated by the pixel defining layer PDL, and provided by being patterned inside the opening OH defined in the pixel defining layer PDL.

In an embodiment, the hole transport region HTR, the emission layer EML-R, EML-G, and EML-B, the electron transport region ETR, etc. of the light emitting elements OEL-1, OEL-2, and OEL-3 may be provided using various methods such as a vacuum deposition method, a spin coating method, a cast method, a Langmuir-Blodgett (LB) method, an inkjet printing method, a laser printing method, and a laser induced thermal imaging (LITI) method.

An encapsulation layer TFE may cover the light emitting elements OEL-1, OEL-2, and OEL-3. The encapsulation layer TFE may seal the display element layer DP-OEL. The encapsulation layer TFE may be disposed on the second electrode EL2, and fill the opening OH.

The encapsulation layer TFE may be a thin film encapsulation layer. The encapsulation layer TFE may be one layer or a laminated layer of multiple layers. The encapsulation layer TFE includes at least one insulating layer. The encapsulation layer TFE according to an embodiment of the inventive concept may include at least one inorganic film (hereinafter, an encapsulating inorganic film). The encapsulation layer TFE according to an embodiment of the inventive concept may include at least one organic film (hereinafter, an encapsulating organic film) and at least one encapsulating inorganic film.

The encapsulating inorganic film protects the display element layer DP-OEL from moisture/oxygen, and the encapsulating organic film protects the display element layer DP-OEL from foreign substances such as dust particles. The encapsulating inorganic film may include a silicon nitride layer, a silicon oxynitride layer, a silicon oxide layer, a titanium oxide layer, an aluminum oxide layer, etc., but is not limited thereto. The encapsulating organic film may include an acryl-based organic film but is not limited thereto.

Although not shown, a capping layer may be further disposed on the second electrode EL2 in an embodiment. For example, the capping layer (not shown) may be disposed between the second electrode EL2 and the encapsulation layer TFE.

Referring to FIGS. 2 and 3, the display device DD may include a non-light emitting region NPXA and light emitting regions PXA-B, PXA-G and PXA-R. Each of the light emitting regions PXA-B, PXA-G and PXA-R may be a region emitting light generated from the light emitting elements ED-1, ED-2, and ED-3, respectively. The light emitting regions PXA-B, PXA-G and PXA-R may be spaced apart from each other on a plane.

Each of the light emitting regions PXA-R, PXA-G and PXA-B may be a region separated by a pixel defining layer PDL. The non-light emitting regions NPXA may be regions between neighboring light emitting regions PXA-R, PXA-G and PXA-B, and may correspond to the pixel defining layer PDL. Each of the light emitting regions PXA-B, PXA-G and PXA-R may correspond to a pixel. The pixel defining layer PDL may separate the light emitting elements OEL-1, OEL-2, and OEL-3. The emission layers EML-R, EML-G and EML-B of the light emitting elements OEL-1, OEL-2, and OEL-3 may be disposed and separated in the opening OH defined in the pixel defining layer PDL. The emission layers EML-R, EML-G, and EML-B separated by the pixel defining layer PDL may be formed through a method such as inkjet printing.

The pixel defining layer PDL may be formed of a polymer resin. For example, the pixel defining layer PDL may be formed including a polyacrylate-based resin or a polyimide-based resin. The pixel defining layer PDL may be formed by further including an inorganic material in addition to the polymer resin. The pixel defining layer PDL may be formed including a light absorbing material, or may be formed including a black pigment or a black dye. The pixel defining layer PDL formed including a black pigment or a black dye may implement a black pixel defining film. When forming the pixel defining layer PDL, carbon black may be used as the black pigment or the black dye, but the embodiment of the inventive concept is not limited thereto.

The pixel defining layer PDL may be formed of an inorganic material. For example, the pixel defining layer PDL may be formed including silicon nitride (SiNx), silicon oxide (SiOx), silicon oxynitride (SiOxNy), etc. The pixel defining layer PDL may define light emitting regions PXA-R, PXA-G, and PXA-B. The light emitting regions PXA-R, PXA-G, and PXA-B and a non-light emitting region NPXA may be separated by the pixel defining layer PDL.

The light emitting regions PXA-R, PXA-G and PXA-B may be divided into groups according to the color of light generated from the light emitting elements OEL-1, OEL-2, and OEL-3. In the display device DD of an embodiment illustrated in FIGS. 2 and 3, three light emitting regions PXA-R, PXA-G and PXA-B which emit red light, green light, and blue light, respectively are illustrated. For example, the display device DD of an embodiment may include a red light emitting region PXA-R, a green light emitting region PXA-G and a blue light emitting region PXA-B, which are distinguished from each other.

The display device DD according to an embodiment includes light emitting elements OEL-1, OEL-2, and OEL-3, and the light emitting elements OEL-1, OEL-2, and OEL-3 may emit light in different wavelength ranges. For example, in an embodiment, the display device DD may include a first light emitting element OEL-1 emitting red light, a second light emitting element OEL-2 emitting green light, and a third light emitting element OEL-3 emitting blue light. However, the embodiment of the inventive concept is not limited thereto, and the first to third light emitting elements OEL-1, OEL-2, and OEL-3 may emit light in a same wavelength range or emit light in at least one different wavelength range. For example, the blue light emitting region PXA-B, the green light emitting region PXA-G, and the red light emitting region PXA-R of the display device DD may correspond to the first light emitting element OEL-1, the second light emitting element OEL-2, and the third light emitting element OEL-3, respectively.

In an embodiment, the first to third light emitting elements OEL-1, OEL-2, and OEL-3 may all emit light in a blue wavelength range. The display device DD may further include a color control layer on an upper part of the display element layer DP-OEL. The color control layer may be a portion that transmits light provided by the light emitting elements OEL-1, OEL-2, and OEL-3 or converts wavelength.

Referring to FIG. 2, the blue light emitting regions PXA-B and the red light emitting regions PXA-R may be alternately arranged in a first direction axis DR1 to form a first group PXG1. The green light emitting regions PXA-G may be arranged in the first direction axis DR1 to form a second group PXG2. The first group PXG1 may be disposed to be spaced apart from the second group PXG2 in a second direction axis DR2. The first group PXG1 and the second group PXG2 each may be provided in plural. The first groups PXG1 and the second groups PXG2 may be alternately arranged in the second direction axis DR2.

One green light emitting region PXA-G may be disposed to be spaced apart from one blue light emitting region PXA-B or one red light emitting region PXA-R in a fourth direction axis DR4. The fourth direction axis DR4 may be a direction between the first direction axis DR1 and the second direction axis DR2.

The arrangement structure of the light emitting regions PXA-B, PXA-G, and PXA-R shown in FIG. 2 may have a pantile structure. However, the arrangement structure of the light emitting regions PXA-B, PXA-G, and PXA-R in the display device DD according to an embodiment is not limited to the arrangement structure shown in FIG. 2. For example, in an embodiment, the light emitting regions PXA-B, PXA-G, and PXA-R may have a stripe structure where the blue light emitting region PXA-B, the green light emitting region PXA-G, and the red light emitting region PXA-R are alternately arranged in the first direction axis DR1.

FIG. 4 is a schematic cross-sectional view of a light emitting element according to an embodiment. FIG. 5 is a schematic cross-sectional view illustrating a part of a light emitting element according to an embodiment. FIG. 5 is a schematic cross-sectional view illustrating a part corresponding to region AA in FIG. 4. As described above, each of the light emitting elements OEL-1, OEL-2, and OEL-3 included in the display device DD shown in FIG. 3, etc. has a structure of a light emitting element OEL shown in FIGS. 4 and 5.

The light emitting element OEL of an embodiment includes a first electrode EL1, a hole transport region HTR disposed over the first electrode EL1, an emission layer EML disposed over the hole transport region HTR, an electron transport region ETR disposed over the emission layer EML, and a second electrode EL2 disposed over the electron transport region ETR. In the light emitting device OEL of an embodiment, the hole transport region may include a first hole transport layer HTL1 disposed adjacent to the first electrode EL1, a second hole transport layer HTL2 disposed adjacent to the emission layer EML, and a third hole transport layer HTL3 disposed between the first hole transport layer HTL1 and the second hole transport layer HTL2.

In an embodiment, the first hole transport layer HTL1 and the second hole transport layer HTL2 may be layers having a smaller refractive index than the third hole transport layer HTL3. The first refractive index of the first hole transport layer HTL1 is less than the third refractive index of the third hole transport layer HTL3, and the second refractive index of the second hole transport layer HTL2 is less than the third refractive index of the third hole transport layer HTL3.

In the light emitting element OEL of an embodiment, the first electrode EL1 has conductivity. The first electrode EL1 may be formed of a metal alloy or a conductive compound. The first electrode EL1 may be an anode. The first electrode EL1 may be a pixel electrode. The first electrode EL1 may be a reflective electrode. When the first electrode EL1 is the reflective electrode, the first electrode EL1 may include Ag, Mg, Cu, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, Li, Ca, LiF/Ca, LiF/Al, Mo, Ti, a compound thereof, or a mixture thereof (e.g., a mixture of Ag and Mg). In an embodiment, the first electrode EL1 may have a structure in which multiple layers are stacked. When the first electrode EL1 has a structure in which multiple layers are stacked, at least one layer may be a reflective film formed of a reflective electrode material. When the first electrode EL1 has a structure in which multiple layers are stacked, at least one layer may include a transparent conductive film formed of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium tin zinc oxide (ITZO), etc. For example, the first electrode EL1 may have a three-layer structure of ITO/Ag/ITO, but it is not limited thereto. A thickness of the first electrode EL1 may be in a range of about 1,000 Å to about 10,000 Å. For example, the thickness of the first electrode EL1 may be in a range of about 1,000 Å to about 3,000 Å.

The hole transport region HTR is provided on the first electrode EL1. The hole transport region HTR may include first to third hole transport layers HTL1, HTL2, and HTL3. The first hole transport layer HTL1 may be disposed under the third hole transport layer HTL3 and the second hole transport layer HTL2 may be disposed over the third hole transport layer HTL3 with respect to the third hole transport layer HTL3 having a relatively greater refractive index compared to other hole transport layers HTL1 and HTL2. In the light emitting element OEL of an embodiment, the hole transport region HTR may include hole transport layers HTL1, HTL2, and HTL3 arranged in the order of a low refractive hole transport layer/high refractive hole transport layer/low refractive hole transport layer in the thickness direction.

A difference between the first refractive index of the first hole transport layer HTL1 and the third refractive index of the third hole transport layer HTL3 may be greater than about 0.1. For example, a difference between the first refractive index and the third refractive index may be greater than or equal to about 0.2. A difference between the second refractive index of the second hole transport layer HTL2 and the third refractive index of the third hole transport layer HTL3 may be greater than about 0.1. For example, a difference between the second refractive index and the third refractive index may be greater than or equal to about 0.2.

The first refractive index of the first hole transport layer HTL1 may be in a range of about 1.2 to about 1.7, and the second refractive index of the second hole transport layer HTL2 may be in a range of about 1.2 to about 1.7. The third refractive index of the third hole transport layer HTL3 may be in a range of about 1.7 to about 2.2. For example, the first refractive index of the first hole transport layer HTL1 may be in a range of about 1.4 to about 1.6, and the second refractive index of the second hole transport layer HTL2 may be in a range of about 1.4 to about 1.6, and the third refractive index of the third hole transport layer HTL3 may be in a range of about 1.9 to about 2.0.

A thickness of the hole transport region HTR may be in a range of about 50 Å to about 15000 Å. For example, the thickness of the hole transport region HTR may be in a range of about 100 Å to about 5000 Å. The thickness ratio (D1:D3:D2) of a thickness of the first hole transport layer HTL1 to a thickness of the third hole transport layer HTL3 to a thickness of the second hole transport layer HTL2 included in the hole transport region HTR may be in a range of about 0.1:0.8:0.1 to about 0.45:0.1:0.45. For example, in an embodiment, the thickness D1 of the first hole transport layer and the thickness D2 of the second hole transport layer are substantially equal, and the thickness D3 of the third hole transport layer may be different from the thickness D1 of the first hole transport layer and the thickness D2 of the second hole transport layer. However, the embodiment of the inventive concept is not limited thereto, and the thickness D1 of the first hole transport layer and the thickness D2 of the second hole transport layer may be different from each other. The thickness ratio (D1:D3:D2) of the thickness of the first hole transport layer HTL1 to the thickness of the third hole transport layer HTL3 to the thickness second hole transport layer HTL2 may be optimally adjusted according to wavelength range of light emitted from an emission layer EML, display quality required in the display device DD (FIG. 2), and the type of a hole transport material used in each of the hole transport layers HTL1, HTL2, and HTL3 of the hole transport region HTR.

For example, in the light emitting element OEL of an embodiment, when the emission layer EML emits blue light having a central wavelength in a wavelength range of about 430 nm to about 470 nm, the thickness ratio (D1:D3:D2) of the thickness of the first hole transport layer HTL1 to the thickness of the third hole transport layer HTL3 to the thickness of the second hole transport layer HTL2 may be about 1:1:1.

The light emitting element OEL of an embodiment may include hole transport layers HTL1, HTL2, and HTL3 arranged in the order of a low refractive hole transport layer/high refractive hole transport layer/low refractive index hole transport layer, thereby achieving improved luminous efficiency characteristics. The light emitting element OEL of an embodiment may include the hole transport layers HTL1, HTL2, and HTL3 of the hole transport region HTR having different refractive indices to minimize light emitted from functional layers inside to disappear due to destructive interference, and to create constructive interference by the hole transport layers HTL1, HTL2, and HTL3 having different refractive indices, thereby achieving high light extraction efficiency.

In an embodiment, the first hole transport layer HTL1 may be directly disposed over the first electrode EL1. The second hole transport layer HTL2 may be directly disposed under the emission layer EML.

In the description, “directly disposed” may mean that there is no layer, film, region, plate or the like added between a portion of a layer, a film, a region, a plate or the like and other portions. For example, “directly disposed” means disposing without additional members, such as an adhesive member between two layers.

In the light emitting element OEL of an embodiment, the refractive index of the first electrode EL1 may be in a range of about 1.7 to about 2.2. For example, the refractive index of the first electrode EL1 may be in a range of about 1.9 to about 2.0. For example, the refractive index of the first electrode EL1 may be greater than the first refractive index of the first hole transport layer HTL1, and a difference in the refractive index between the first hole transport layer HTL1 and the first electrode ELL which are adjacent to each other, may be greater than about 0.1.

In the light emitting element OEL of an embodiment, the refractive index of the emission layer EML may be in a range of about 1.7 to about 2.2. For example, the refractive index of the emission layer EML may be in a range of about 1.9 to about 2.0. For example, the refractive index of the emission layer EML may be greater than the second refractive index of the second hole transport layer HTL2, and a difference in the refractive index between the second hole transport layer HTL2 and the emission layer EML, which are adjacent to each other, may be greater than about 0.1.

For example, the light emitting element OEL of an embodiment includes a hole transport region HTR, in which hole transport layers HTL1 and HTL2 having different refractive indices from the adjacent first electrode EL1 or the emission layer EML are disposed, to achieve light extraction efficiency characteristics and improved luminous efficiency characteristics.

The first hole transport layer HTL1 and the second hole transport layer HTL2 may each independently include at least one of the compounds represented by Formulas 1-1 to 1-4 below. The compounds represented by Formulas 1-1 to 1-4 may have a refractive index in a range of about 1.2 to about 1.7. The first hole transport layer HTL1 and the second hole transport layer HTL2 each may each independently be formed of any one of the compounds represented by Formulas 1-1 to 1-4, or mixtures thereof.

In Formulas 1-1 to 1-4 above, A₁ to A5 may be each independently a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, a substituted or unsubstituted silyl group, a substituted or unsubstituted oxy group, a substituted or unsubstituted alkyl group having 1 to 60 carbon atoms, a substituted or unsubstituted aryl group having 6 to 60 ring-forming carbon atoms, a substituted or unsubstituted heteroaryl group having 2 to 60 ring-forming carbon atoms. In Formula 1-2, a may be an integer from 0 to 5, and in Formula 1-3, b may be an integer from 0 to 4, and in Formula 1-4, c may be an integer from 0 to 6.

In the description, the term “substituted or unsubstituted” may indicate that one is substituted or unsubstituted with at least one substituent selected from the group consisting of a deuterium atom, a halogen atom, a cyano group, a nitro group, an amino group, a silyl group, oxy group, thio group, sulfinyl group, sulfonyl group, carbonyl group, a boron group, a phosphine oxide group, a phosphine sulfide group, an alkyl group, an alkenyl group, an alkynyl group, an alkoxy group, a hydrocarbon ring group, an aryl group, and a heterocyclic group. Each of the substituents recited above may be substituted or unsubstituted. For example, a biphenyl group may be interpreted as an aryl group or a phenyl group substituted with a phenyl group.

In the description, the term “bonded to an adjacent group to form a ring” may indicate that one is bonded to an adjacent group to form a substituted or unsubstituted hydrocarbon ring, or a substituted or unsubstituted heterocycle. The hydrocarbon ring includes an aliphatic hydrocarbon ring and an aromatic hydrocarbon ring. The heterocycle includes an aliphatic heterocycle and an aromatic heterocycle. Rings formed by being bonded to an adjacent group may be monocyclic or polycyclic. The rings formed by being bonded to each other may be connected to another ring to form a spiro structure.

In the description, the term “an adjacent group” may mean a substituent substituted for an atom which is directly connected to an atom substituted with a corresponding substituent, another substituent substituted for an atom which is substituted with a corresponding substituent, or a substituent sterically positioned at the nearest position to a corresponding substituent. For example, two methyl groups in 1,2-dimethylbenzene may be interpreted as mutually “adjacent groups” and two ethyl groups in 1,1-diethylcyclopentane may be interpreted as mutually “adjacent groups”.

In the description, examples of a halogen atom may include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom.

In the description, an alkyl group may be a linear, branched, or cyclic type. The number of carbon atoms in the alkyl group is 1 to 50, 1 to 30, 1 to 20, 1 to 10, or 1 to 6. Examples of the alkyl group may include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a s-butyl group, a t-butyl group, an i-butyl group, a 2-ethylbutyl group, a 3,3-a dimethylbutyl group, an n-pentyl group, an i-pentyl group, a neopentyl group, a t-pentyl group, a cyclopentyl group, a 1-methylpentyl group, a 3-methylpentyl group, a 2-ethylpentyl group, a 4-methyl-2-pentyl group, an n-hexyl group, a 1-methylhexyl group, a 2-ethylhexyl group, a 2-butylhexyl group, a cyclohexyl group, a 4-methylcyclohexyl group, a 4-t-butylcyclohexyl group, an n-heptyl group, a 1-methylheptyl group, a 2,2-dimethylheptyl group, a 2-ethylheptyl group, a 2-butylheptyl group, an n-octyl group, a t-octyl group, a 2-ethyloctyl group, a 2-butyloctyl group, a 2-hexyloctyl group, a 3,7-dimethyloctyl group, a cyclooctyl group, an n-nonyl group, an n-decyl group, an adamantyl group, a 2-ethyldecyl group, a 2-butyldecyl group, a 2-hexyldecyl group, a 2-octyldecyl group, an n-undecyl group, an n-dodecyl group, a 2-ethyldodecyl group, a 2-butyldodecyl group, a 2-hexyldocecyl group, a 2-octyldodecyl group, an n-tridecyl group, an n-tetradecyl group, an n-pentadecyl group, an n-hexadecyl group, a 2-ethylhexadecyl group, a 2-butylhexadecyl group, a 2-hexylhexadecyl group, a 2-octylhexadecyl group, an n-heptadecyl group, an n-octadecyl group, an n-nonadecyl group, an n-eicosyl group, a 2-ethyleicosyl group, a 2-butyleicosyl group, a 2-hexyleicosyl group, a 2-octyleicosyl group, an n-henicosyl group, an n-docosyl group, an n-tricosyl group, an n-tetracosyl group, an n-pentacosyl group, an n-hexacosyl group, an n-heptacosyl group, an n-octacosyl group, an n-nonacosyl group, an n-triacontyl group, etc., but are not limited thereto.

In the description, an aryl group means any functional group or substituent derived from an aromatic hydrocarbon ring. The aryl group may be a monocyclic aryl group or a polycyclic aryl group. The number of ring-forming carbon atoms in the aryl group may be 6 to 30, 6 to 20, or 6 to 15. Examples of the aryl group may include a phenyl group, a naphthyl group, a fluorenyl group, an anthracenyl group, a phenanthryl group, a biphenyl group, a terphenyl group, a quaterphenyl group, a quinquephenyl group, a sexiphenyl group, a triphenylenyl group, a pyrenyl group, a benzofluoranthenyl group, a chrysenyl group, etc., but are not limited thereto.

In the description, a fluorenyl group may be substituted, and two substituents may be bonded to each other to form a spiro structure. An example that the fluorenyl group is substituted is as follows. However, the embodiment of the inventive concept is not limited thereto.

In the description, a heteroaryl group may include at least one of B, O, N, P, Si, or S as a hetero atom. When the heteroaryl group contains two or more hetero atoms, the two or more hetero atoms may be the same as or different from each other. The heteroaryl group may be a monocyclic heteroaryl group or a polycyclic heteroaryl group. The number of ring-forming carbon atoms in the heteroaryl group may be 2 to 30, 2 to 20, or 2 to 10. Examples of the heteroaryl group may include a thiophene group, a furan group, a pyrrole group, an imidazole group, a triazole group, a pyridine group, a bipyridine group, a pyrimidine group, a triazine group, a triazole group, an acridyl group, a pyridazine group, a pyrazinyl group, a quinoline group, a quinazoline group, a quinoxaline group, a phenoxazine group, a phthalazine group, a pyrido pyrimidine group, a pyrido pyrazine group, a pyrazino pyrazine group, an isoquinoline group, an indole group, a carbazole group, an N-arylcarbazole group, an N-heteroarylcarbazole group, an N-alkylcarbazole group, a benzoxazole group, a benzoimidazole group, a benzothiazole group, a benzocarbazole group, a benzothiophene group, a dibenzothiophene group, a thienothiophene group, a benzofuran group, a phenanthroline group, a thiazole group, an isoxazole group, an oxazole group, an oxadiazole group, a thiadiazole group, a phenothiazine group, a dibenzosilole group, a dibenzofuran group, etc., but are not limited thereto.

In the description, the above description on the aryl group may be applied to an arylene group, except that the arylene group is a divalent group. The above description on the heteroaryl group may be applied to a heteroarylene group, except that the heteroarylene group is a divalent group.

In the description, a silyl group includes an alkyl silyl group and an aryl silyl group. Examples of the silyl group include a trimethylsilyl group, a triethylsilyl group, a t-butyldimethylsilyl group, a vinyldimethylsilyl group, a propyldimethylsilyl group, a triphenylsilyl group, a diphenylsilyl group, a phenylsilyl group, etc., but are not limited thereto.

In the description, the number of carbon atoms in an amino group is not particularly limited, but may be 1 to 30. The amino group may include an alkyl amino group, an aryl amino group, or a heteroaryl amino group. Examples of the amino group include a methylamino group, a dimethylamino group, a phenylamino group, a diphenylamino group, a naphthylamino group, a 9-methyl-anthracenylamino group, a triphenylamino group, etc., but are not limited thereto.

The third hole transport layer HTL3 may include a compound represented by Formula 2 below. The compound represented by Formula 2 may have a refractive index in a range of about 1.7 to about 2.2.

In Formula 2 above, Ar₁ and Ar₂ may be each independently a hydrogen atom, a deuterium atom, a halogen atom, a substituted or unsubstituted alkyl group having 1 to 30 carbon atoms, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, a substituted or unsubstituted heteroaryl group having 2 to 30 ring-forming carbon atoms, or bonded to an adjacent group to form a ring. Ara may be a substituted or unsubstituted aryl group having 6 to 30 ring-forming carbon atoms or a substituted or unsubstituted heteroaryl group having 2 to 30 ring-forming carbon atoms. In Formula 2, a and b may be each independently 0 or 1, and L₁ and L₂ may be each independently a substituted or unsubstituted cycloalkylene group having 3 to 10 ring-forming carbon atoms, a substituted or unsubstituted heterocycloalkylene group having 2 to 10 ring-forming carbon atoms, a substituted or unsubstituted cycloalkenylene group having 3 to 10 ring-forming carbon atoms, a substituted or unsubstituted arylene group having 6 to 30 ring-forming carbon atoms, or a substituted or unsubstituted heteroarylene group having 2 to 60 ring-forming carbon atoms. In Formula 2, p and s may be each independently an integer from 0 to 4, q and r may be each independently an integer from 0 to 3, and R₁ to R₅ may be each independently a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, a substituted or unsubstituted silyl group, a substituted or unsubstituted oxy group, a substituted or unsubstituted alkyl group having 1 to 60 carbon atoms, a substituted or unsubstituted heterocycloalkyl group having 3 to 60 ring-forming carbon atoms, a substituted or unsubstituted aryl group having 6 to 60 ring-forming carbon atoms, or a substituted or unsubstituted heteroaryl group having 2 to 60 ring-forming carbon atoms.

The compound for the third hole transport layer HTL3 represented by Formula 2 may be represented by any one of the compounds of Compound Group 1 below, which contains Compounds 1 to 69. The third hole transport layer HTL3 in the light emitting element OEL of an embodiment may include at least one of Compounds 1 to 69 of Compound Group 1.

The hole transport region HTR of the light emitting element OEL of an embodiment may only include three hole transport layers HTL1, HTL2, and HTL3. The light emitting element OEL of an embodiment may include hole transport layers in which the first hole transport layer HTL1/third hole transport layer HTL3/second hole transport layer HTL2 are stacked in order between the first electrode EL1 and the emission layer EML, thereby achieving excellent luminous efficiency characteristics. In an embodiment, the refractive indices of the first hole transport layer HTL1 and the second hole transport layer HTL2 are less than the refractive index of the third hole transport layer HTL3, and a difference in the refractive index may be greater than about 0.1.

The emission layer EML is provided on the hole transport region HTR. A thickness of the emission layer EML may be, for example, in a range of about 100 Å to about 1000 Å. For example, the thickness of the emission layer EML may be in a range of about 100 Å to about 300 Å. The emission layer EML may have a single layer formed of a single material, a single layer formed of different materials, or a multilayer structure having multiple layers formed of different materials.

The emission layer EML may emit one of red, green, blue, white, yellow, or cyan light. The emission layer EML may include a fluorescence light emitting material or a phosphorescence light emitting material. In an embodiment, the emission layer EML may include quantum dots.

In the light emitting element OEL of an embodiment, the emission layer EML may include an anthracene derivative, a pyrene derivative, a fluoranthene derivative, a chrysene derivative, a dihydrobenzanthracene derivative, a triphenylene derivative, etc. For example, the emission layer EML may include an anthracene derivative, a pyrene derivative, etc. However, the embodiment of the inventive concept is not limited thereto, and the emission layer EML may include a known light emitting material.

In the light emitting element OEL of an embodiment, the electron transport region ETR is provided on the emission layer EML. The electron transport region ETR may include at least one of a hole blocking layer, an electron transport layer, or an electron injection layer, but the embodiment of the inventive concept is not limited thereto.

The electron transport region ETR may have a single layer formed of a single material, a single layer formed of different materials, or a multilayer structure including layers formed of different materials.

For example, the electron transport region ETR may have a single layer structure of an electron injection layer EIL or an electron transport layer ETL, and may have a single layer structure formed of an electron injection material and an electron transport material. The electron transport region ETR may have a single layer structure formed of different materials, or may have a structure in which an electron transport layer/electron injection layer and a hole blocking layer/electron transport layer/electron injection layer are stacked in order from the emission layer EML, but is not limited thereto. The thickness of the electron transport region ETR may be, for example, in a range of about 1,000 Å to about 1,500 Å.

When the electron transport region ETR includes the electron injection layer, the electron transport region ETR may be a halogenated metal such as LiF, NaCl, CsF, RbCl, RbI, and Cul, a lanthanide metal such as Yb, a metal oxide such as Li₂O, BaO, or lithium quinolate (LiQ), but is not limited thereto. The electron injection layer may also be formed of a mixture material of an electron transport material and an insulating organo-metal salt. The organo-metal salt may be a material having an energy band gap of greater than or equal to about 4 eV. For example, the organo-metal salt may include, metal acetates, metal benzoates, metal acetoacetates, metal acetylacetonates, or metal stearates. When the electron transport region ETR includes the electron transport layer, the electron transport region ETR may include an anthracene-based compound. However, the embodiment of the inventive concept is not limited thereto, and the electron transport region ETR may include a known electron transport material.

The second electrode EL2 is provided on the electron transport region ETR. The second electrode EL2 may be a common electrode or a cathode. The second electrode EL2 may be a transmissive electrode or a transflective electrode. If the second electrode EL2 is the transmissive electrode, the second electrode EL2 may include a transparent metal oxide, for example, indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium tin zinc oxide (ITZO), etc. If the second electrode EL2 is the transflective electrode, the second electrode EL2 may include Ag, Mg, Cu, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, Li, Ca, LiF/Ca, LiF/Al, Mo, Ti, a compound thereof, or a mixture thereof (e.g., a mixture of Ag and Mg). The second electrode EL2 may have a multilayer structure including a reflective film or a transflective film formed of the above-described materials, and a transparent conductive film formed of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium tin zinc oxide (ITZO), etc.

A capping layer (not shown) may be disposed on the second electrode EL2 of the light emitting element OEL of an embodiment. The capping layer (not shown) may include, for example, α-NPD, NPB, TPD, m-MTDATA, Alq₃, CuPc, N4,N4,N4′,N4′-tetra(biphenyl-4-yl) biphenyl-4,4′-diamine (TPD15), 4,4′,4″-tris(carbazol sol-9-yl)triphenylamine (TCTA), N, N′-bis(naphthalen-1-yl), etc.

A display device of an embodiment may include light emitting elements, and at least one light emitting element among the light emitting elements may have a configuration of the light emitting element according to an embodiment described above.

Referring back to FIG. 3, the display device DD of an embodiment includes first to third light emitting elements OEL-1, OEL-2, and OEL-3 separated by a pixel defining layer PDL and the first to third light emitting elements OEL-1, OEL-2, and OEL-3 have different configurations of emission layers EML-B, EML-G, and EML-R, and thus may emit light in different wavelength ranges. One of the first to third light emitting elements OEL-1, OEL-2, and OEL-3 may have the configuration of the light emitting element of FIGS. 4 and 5 described above. In an embodiment, two light emitting elements selected from the first to third light emitting elements OEL-1, OEL-2, and OEL-3 or all three light emitting elements may have the configuration of the light emitting element of FIGS. 4 and 5 described above.

When all three light emitting elements OEL-1, OEL-2, and OEL-3 in the display device DD of an embodiment have the configuration of the light emitting element of FIGS. 4 and 5 described above, the hole transport region HTR may be provided as a common layer in all of the first to third light emitting elements OEL-1, OEL-2, and OEL-3. For example, the hole transport region HTR provided as the common layer may have a structure including the first to third hole transport layers HTL1, HTL2, and HTL3.

In the display device of an embodiment, unlike the one illustrated in FIG. 3, the hole transport region HTR is disposed in an opening OH defined in the pixel defining layer PDL, and may be provided to be separated to correspond to the emission layers EML-B, EML-G, and EML-R. The hole transport regions HTR included in each of the light emitting elements OEL-1, OEL-2, and OEL-3 have a structure including the first to third hole transport layers HTL1, HTL2, and HTL3. When the hole transport region HTR is provided to be separated to correspond not to the common layer but to the light emitting elements OEL-1, OEL-2, and OEL-3, the thickness ratio of the first to third hole transport layers HTL1, HTL2, and HTL3 included in each of the light emitting element OEL-1, OEL-2, and OEL-3 may be controlled to vary according to the wavelength range of light emitted from each of the light emitting elements OEL-1, OEL-2, and OEL-3.

In the display device DD of another embodiment, the first light emitting element OEL-1 emitting blue light may have a light emitting element structure including first to third hole transport layers HTL1, HTL2, and HTL3. However, the embodiment of the inventive concept is not limited thereto.

FIG. 6 is a graph showing comparison of luminous efficiency of Comparative Examples and Examples. Example shows evaluation results for light emitting elements having a hole transport region structure of the light emitting element of an embodiment described above, and Comparative Examples 1 to 4 show evaluation results for light emitting elements having a hole transport region configuration which is different from Example. The configuration of other functional layers of the light emitting elements was the same in Comparative Examples and Example, except the configuration of the hole transport region. Comparative Examples and Example are light emitting elements emitting blue light having a central wavelength in the vicinity of 464 nm.

Comparative Examples 1 and 2 are the cases in which the hole transport region is formed of one hole transport layer, respectively. Comparative Example 1 is the case of including only one hole transport layer having a refractive index of about 1.9, and Comparative Example 2 is the case of including only one hole transport layer having a refractive index of about 1.4.

Comparative Examples 3 and 4 are the cases where the hole transport region is formed of two-layered hole transport layers, respectively. In Comparative Example 3, the refractive index of the hole transport layer adjacent to the first electrode was about 1.4, and the refractive index of the hole transport layer adjacent to the emission layer was about 1.9. In Comparative Example 4, the refractive index of the hole transport layer adjacent to the first electrode was about 1.9, and the refractive index of the hole transport layer adjacent to the emission layer was about 1.4. For example, Comparative Examples 3 and 4 are the cases where the stacking order of the low-refractive hole transport layer and the high-refractive hole transport layer is different.

Example has the structure of the hole transport region of the light emitting element described above, which is the case where three-layered hole transport layers are included, the refractive indices of the first hole transport layer adjacent to the first electrode and the second hole transport layer adjacent to the emission layer are about 1.4, respectively, the refractive index of the third hole transport layer disposed between the first hole transport layer and the second hole transport layer is about 1.9.

In FIG. 6, the horizontal axis refers to a color coordinate value and corresponds to a “y” value of the color coordinate of light emitted from a light emitting element. The value shown in the horizontal axis in FIG. 6 corresponds to a y value in CIE color coordinates. The graph of FIG. 6 shows luminous efficiency according to the color coordinate of emitted light. Referring to the results of FIG. 6, it is seen that in a color coordinate value of 0.04 to 0.1, the light emitting element of Example showed higher luminous efficiency than those of Comparative Example. Example showed improved luminous efficiency of about 34% compared to Comparative Example 1.

The light emitting element of an embodiment includes a hole transport region having a stacked structure of a low-refractive hole transport layer/high-refractive hole transport layer/low-refractive hole transport layer to exhibit a high light extraction effect, thereby achieving excellent luminous efficiency characteristics. The di splay device of an embodiment may include a light emitting element having a hole transport region in which hole transport layers having different refractive indices are stacked, thereby having high luminance characteristics.

A light emitting element of an embodiment may achieve improved light extraction characteristics by including hole transport layers having different refractive indices.

A light emitting element of an embodiment may achieve excellent light luminous efficiency by including a light emitting element containing hole transport layers having different refractive indices.

Although the inventive concept has been described with reference to embodiments of the inventive concept, it will be understood that the inventive concept should not be limited to these embodiments, but various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the inventive concept.

Accordingly, the technical scope of the inventive concept is not intended to be limited to the contents set forth in the detailed description of the specification, but is intended to be defined by the appended claims. 

What is claimed is:
 1. A light emitting element comprising: a first electrode; a hole transport region disposed over the first electrode; an emission layer disposed over the hole transport region; an electron transport region disposed over the emission layer; and a second electrode disposed over the electron transport region, wherein the hole transport region comprises: a first hole transport layer disposed adjacent to the first electrode and having a first refractive index; a second hole transport layer disposed adjacent to the emission layer and having a second refractive index; and a third hole transport layer disposed between the first hole transport layer and the second hole transport layer and having a third refractive index greater than each of the first refractive index and the second refractive index.
 2. The light emitting element of claim 1, wherein a difference between the third refractive index and the first refractive index is greater than about 0.1, and a difference between the third refractive index and the second refractive index is greater than about 0.1.
 3. The light emitting element of claim 2, wherein the first refractive index is in a range of about 1.2 to about 1.7, the second refractive index is in a range of about 1.2 to about 1.7, and the third refractive index is in a range of about 1.7 to about 2.2.
 4. The light emitting element of claim 2, wherein the first refractive index and the second refractive index are equal.
 5. The light emitting element of claim 1, wherein the second hole transport layer is directly disposed under the emission layer.
 6. The light emitting element of claim 5, wherein a refractive index of the emission layer is greater than the first refractive index of the first hole transport layer, and a difference between the refractive index of the emission layer and the first refractive index is greater than about 0.1.
 7. The light emitting element of claim 6, wherein the refractive index of the emission layer is in a range of about 1.7 to about 2.2.
 8. The light emitting element of claim 1, wherein the first hole transport layer is directly disposed over the first electrode.
 9. The light emitting element of claim 8, wherein a refractive index of the first electrode is greater than the first refractive index of the first hole transport layer, and a difference between the refractive index of the first electrode and the first refractive index is greater than about 0.1.
 10. The light emitting element of claim 9, wherein the refractive index of the first electrode is in a range of about 1.7 to about 2.2.
 11. The light emitting element of claim 1, wherein a ratio of a thickness of the first hole transport layer to a thickness of the third hole transport layer to a thickness of the second hole transport layer is in a range of about 0.1:0.8:0.1 to about 0.45:0.1:0.45.
 12. The light emitting element of claim 1, wherein the first electrode is a reflective electrode, and the second electrode is a transmissive electrode or a transflective electrode.
 13. The light emitting element of claim 1, wherein the emission layer emits light having a central wavelength in a range of about 430 nm to about 470 nm.
 14. The light emitting element of claim 13, wherein a ratio of a thickness of the first hole transport layer to a thickness of the third hole transport layer to a thickness of the second hole transport layer is about 1:1:1.
 15. The light emitting element of claim 1, wherein the first hole transport layer and the second hole transport layer each independently comprise at least one of the compounds represented by Formulae 1-1 to 1-4:

wherein in Formulae 1-1 to 1-4, A₁ to A₅ are each independently a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, a substituted or unsubstituted silyl group, a substitution or unsubstituted oxy group, a substituted or unsubstituted alkyl group having 1 to 60 carbon atoms, a substituted or unsubstituted aryl group having 6 to 60 ring-forming carbon atoms, or a substituted or unsubstituted heteroaryl group having 2 to 60 ring-forming carbon atoms, a is an integer from 0 to 5, b is an integer from 0 to 4, and c is an integer from 0 to
 6. 16. The light emitting element of claim 1, wherein the third hole transport layer comprises a compound represented by Formula 2:

wherein in Formula 2, Ar₁ and Ar₂ are each independently a hydrogen atom, a deuterium atom, a halogen atom, a substituted or unsubstituted alkyl group having 1 to 30 carbon atoms, a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, a substituted or unsubstituted heteroaryl group having 2 to 30 ring-forming carbon atoms, or bonded to an adjacent group to form a ring, Ar₃ is a substituted or unsubstituted aryl group having 6 to 30 ring-forming carbon atoms, or a substituted or unsubstituted heteroaryl group having 2 to 30 ring-forming carbon atoms, a and b are each independently 0 or 1, L₁ and L₂ are each independently a substituted or unsubstituted cycloalkylene group having 3 to 10 ring-forming carbon atoms, a substituted or unsubstituted heterocycloalkylene group having 2 to 10 ring-forming carbon atoms, a substituted or unsubstituted cycloalkenylene group having 3 to 10 ring-forming carbon atoms, a substituted or unsubstituted arylene group having 6 to 30 ring-forming carbon atoms, or a substituted or unsubstituted heteroarylene group having 2 to 60 ring-forming carbon atoms, p and s are each independently an integer from 0 to 4, q and r are each independently an integer from 0 to 3, and R₁ to R₅ are each independently a hydrogen atom, a deuterium atom, a halogen atom, a hydroxyl group, a cyano group, a nitro group, an amino group, a substituted or unsubstituted silyl group, a substituted or unsubstituted oxy group, a substituted or unsubstituted alkyl group having 1 to 60 carbon atoms, a substituted or unsubstituted heterocycloalkyl group having 3 to 60 ring-forming carbon atoms, a substituted or unsubstituted aryl group having 6 to 60 ring-forming carbon atoms, or a substituted or unsubstituted heteroaryl group having 2 to 60 ring-forming carbon atoms.
 17. The light emitting element of claim 1, wherein the third hole transport layer comprises at least one of Compounds 1 to 69:


18. A display device comprising: a plurality of light emitting elements, each of the light emitting elements comprising: a first electrode; a hole transport region disposed over the first electrode; an emission layer disposed over the hole transport region; an electron transport region disposed over the emission layer; and a second electrode disposed over the electron transport region, wherein the hole transport region of at least one of the plurality of light emitting elements comprises: a first hole transport layer disposed adjacent to the first electrode and having a first refractive index; a second hole transport layer disposed adjacent to the emission layer and having a second refractive index; and a third hole transport layer disposed between the first hole transport layer and the second hole transport layer and having a third refractive index greater than each of the first refractive index and the second refractive index.
 19. The display device of claim 18, wherein a difference between the third refractive index and the first refractive index is greater than about 0.1, and a difference between the third refractive index and the second refractive index is greater than about 0.1.
 20. The display device of claim 18, wherein the first electrode is a reflective electrode, and the second electrode is a transmissive electrode or a transflective electrode. 